1. Field of the Invention
The present invention relates to a semiconductor memory device and a method for producing the same. More particularly, it relates to a dynamic random access memory (DRAM) cell having a trench capacitor and a method for producing the DRAM cell.
2. Description of the Related Art
Conventionally, to miniaturize a DRAM cell, the cell area is reduced and in order to increase storage capacity of the miniaturized cell, trench capacitors are used to obtain a larger substantial capacitor area than that provided in a conventional plane area.
The above process, however, is disadvantageous in that when the trench capacitors are arranged close to each other, data stored therein is lost due to a punch-through phenomenon occurring between storage electrodes, and in that the capture of carriers generated in a semiconductor substrate by alpha (.alpha.) ray irradiation is large, thereby causing soft errors.
To alleviate the above problems it is necessary to form impurity-implanted regions, having a concentration of an order 1 to 2 degrees higher than the concentration in the substrate, around the trench capacitor, thus decreasing the width of the depletion layer and forcing both capacitors to be located close together. However, in the above, an ion implantation process cannot be effectively used as the process for implanting impurities into the side surfaces of the trench capacitors.
To prevent the occurrence of the above problems, a semiconductor integrated circuit device is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 59-191373 (hereinafter referred to as "the publication-373") filed on May 15, 1983.
According to the structure of the trench capacitor disclosed in the '373 publication, a storage electrode plate and an insulating film for the inner wall of the trench capacitor are formed around the capacitor dielectric film. By forming the insulating film on the inner wall of the trench capacitor, the generation of undesired depression regions, which extend from the respective trench capacitor to the semiconductor substrate and cause operational errors, can be avoided.
However, the above conventional trench capacitors are positioned on each side of a field oxide film having a so-called "bird's beak" formed at both ends thereof. The field oxide film having the bird's beak has minimum a width of about 2 .mu.m, even if the most superior lithography techniques are used in the etching thereof.
Therefore, trench capacitors must be formed in a space having a width of at least 2 .mu.m to accommodate the oxide film. Thus, the structure disclosed in the '373 publication has insufficient miniaturization effects, even though the above punch-through phenomenon is prevented by the formation of the storage electrode plate and the insulating film for the inner wall of the trench.